Magnetic resonance spectroscopy (MRS) is used in e. g. analytical chemistry to retrieve detailed information about chemicals, but also for non-invasive in vivo experiments in biology and medicine to obtain quantitative information about biologically active compounds. Specifically, in the rapidly evolving field of neuroscience, MRS is of great importance, because it allows the assessment of neuro-chemical profiles of individual brain regions.
A non-homogeneous distribution of the magnetic field strength within the sensitive probe volume leads to broadening of spectral peaks and a reduction of the signal-to-noise ratio (SNR). Furthermore, the distribution of the magnetic field has a direct impact on the line shape of spectral peaks, which may lead to quantification errors, if inappropriate model functions are used. If magnetically susceptible probes are placed into the homogeneous field of an MR scanner, the field distribution often becomes inhomogeneous and a local distribution of field gradients is added. By correcting (adjusting) the field distribution, it can be made homogeneous with the probe inside the MR device (‘shimming’), in particular in the axial direction of a gradient device of the MR device (cartesian z-direction).
Shimming can be achieved by placement of configurations of ferromagnetic objects with proper size and positioning into the magnetic field in order to improve the field homogeneity within the sensitive probe volume (so-called ‘passive shimming’). Passive shimming is used e. g. to improve the field homogeneity of the MR scanner and also to account for subject induced field distortions (see e. g. F. Romeo et al. in “Magn. Reson. Med.” 1984. 1(1): p. 44-65; J. V. M. McGinley et al. in U.S. Pat. No. 5,532,597; B. Dorri in U.S. Pat. No. 5,677,854; G. Neuberth in U.S. Pat. No. 6,897,750, A. Jesmanowicz et al. in ISMRM, Annual Meeting 2001, Glasgow/Scotland, and D. F. Hillenbrand et al. in “Appl. Magn. Reson.”, 2005. 29: p. 39-64).
Conventional passive shimming has a drawback in terms of complexity of constructing the ferromagnetic object configurations and flexibility for adapting to varying measurement condition. In particular, the physical mounting of many metal pieces on a carrier is time consuming and inflexible. As an example, the technique of A. Jesmanowicz et al. requires exact mounting of more than 700 metal pieces on a carrier. Therefore, the practical use of passive shimming has been limited to special applications like the use of intra-oral passive shimming of the frontal cortex via diamagnetic mouth inserts (see e. g. J. L. Wilson et al. in “Magn. Reson. Med.” 2002. 48(5): p. 906-14; J. L. Wilson et al. in “Magn. Reson. Med.” 2003, 50(5): p. 1089-94; and R. Cusack et al. in “Neuroimage” 2005, 24(1): p. 82-91).
Furthermore, shimming can be achieved by shim fields generated with electro-magnetic coils arranged around the sensitive probe volume (so-called ‘active shimming’). Active shimming was developed to provide highly accurate field cancellation with a flexible interface. Magnetic fields, as vector fields, can be fully described by spherical harmonic functions. Accordingly, deviations from homogeneity can also be expressed on that basis. With a set of appropriate electomagnetic coils, each generating a magnetic field component that corresponds to one spherical harmonic, the field inhomogeneity can be minimized by superposition of a shim field of the same magnitude but opposite sign to the distortion (see e.g. F. Romeo et al. in “Magn. Reson. Med.” 1984. 1(1): p. 44-65; P. Konzbul et al. in “Meas. Sci. Technol.”, 1995. 6: p. 1116-1123; M. A. Brideson et al. in “Concepts in Magnetic Resonance”, 2001. 14: p. 9-18; and M. A. Brideson et al. in “Meas. Sci. Technol.”, 2003. 14: p. 484-493).
Conventional active shimming has a restriction in high field strength applications. However, there is a continuous trend towards the use of higher magnetic fields for both research and clinical scanners, since the SNR improves at least linearly in MR imaging and spectroscopy with increased magnetic field. Furthermore, higher fields lead to better spectral dispersion in MR spectroscopy, which is advantageous particularly for the separation of glutamate (Glu) from glutamine (Gln) or creatine (Cr) from phosphocreatine (PCr) in 1H MRS of brain metabolites.
However, the benefit of better SNR and simplified spectra at high field can only be exploited if optimal shimming is achieved, as field distortions due to susceptibility effects also increase at higher field strength. Increased field distortions require stronger shim fields, potentially exceeding the capabilities of the conventional active shim devices. In addition, regions of strongly differing magnetic susceptibilities like tissue-air transitions, e.g. in the vicinity of the ear canals or the cranial bone, may also require shim fields beyond those provided by active shimming. In practice, zero order terms, i.e. a field offsets, that are small compared to the scanner field strength do not pose problems and first order terms can be easily shimmed using the scanners gradient system. The second order terms, however, pose the main bottleneck in conventional experimental setups when the shim requirements exceed the capabilities of the corresponding active shim device.